Hubble Space TelescopeObservations of Nine High-Redshift ESSENCE Supernovae

Abstract

We present broadband light curves of nine supernovae ranging in redshift from 0.5 to 0.8. The supernovae were discovered as part of the ESSENCE project, and the light curves are a combination of Cerro Tololo 4 m and Hubble Space Telescope (HST ) photometry. On the basis of spectra and/or light-curve fitting, eight of these objects are definitely Type Ia supernovae, while the classification of one is problematic. The ESSENCE project is a 5 yr endeavor to discover about 200 high-redshift Type Ia supernovae, with the goal of tightly constraining the time average of the equation-ofstate parameter [w ¼ p/(c 2 )] of the ''dark energy.'' To help minimize our systematic errors, all of our ground-based photometry is obtained with the same telescope and instrument. In 2003 the highest redshift subset of ESSENCE supernovae was selected for detailed study with HST. Here we present the first photometric results of the survey. We find that all but one of the ESSENCE supernovae have slowly declining light curves and that the sample is not representative of the low-redshift set of ESSENCE Type Ia supernovae. This is unlikely to be a sign of evolution in the population. We attribute the decline-rate distribution of HSTevents to a selection bias at the high-redshift edge of our sample and find that such a bias will infect other magnitude-limited Type Ia supernova searches unless appropriate precautions are taken.

Nearby Supernova Factory Observations of SN 2005gj: Another Type Ia Supernova in a Massive Circumstellar Envelope

“…Such data usually were not available in the Supernova Cosmology Project or High-Z Supernova Search Team SN Ia cosmology searches of the late 1990s. More modern rolling searches such as the Supernova Legacy Survey (Astier et al 2006) and ESSENCE (Krisciunas et al 2005) contain such data except for SNe Ia discovered late relative to the observing window for a given program field. As the high-redshift SN Ia searches have netted several hundred events to date, events like SN 2005gj and SN 2002ic should have been detected unless their incidence declines strongly with redshift.…”

Supernovae and cosmology

“…All corresponding spectroscopy has been published (Matheson et al 2005, Blondin et al 2006. A first detailed description of photometry of a subset of the ESSENCE events observed with the Hubble Space Telescope (HST) pointed out some potential selection effects in the sample (Krisciunas et al 2005). An evaluation of exotic proposals for dark energy when compared to the available SN Ia data was made in Davis et al (2007).…”

“…First, the SNe Ia are not standard candles and their luminosity function is spanning almost a factor of 10 from the brightest to the faintest events. Even though the most extreme cases are not included the most distant supernovae are also the most luminous ones (Krisciunas et al 2005). Second, supernova searches all use a certain frequency, with which the search fields are monitored.…”

K‐Corrections and Spectral Templates of Type Ia Supernovae

“…Large surveys such as the SuperNova Legacy Survey 5 (Astier et al 2006) and ESSENCE 6 ( Krisciunas et al 2005) aim to constrain the equation-of-state parameter of dark energy with photometric and spectroscopic observations of hundreds of highredshift SNe Ia. With such a large sample size, the systematic uncertainties in K-corrections and photometric calibration become comparable to the intrinsic dispersion in the error budget.…”